Wavescan doesn't know binary bus signals, I think. Your "digital bus signal" is actually a set of digital lines representing a binary code. If you browse the Wavescan calculator features, you surely find out how they can be utilized to make a behavioral DAC model, something like d7 + d6/2 + d5/4 .... d0/256 (a 8-bit DAC example). Can be further imnproved by making bus signals "digital" using sign() or clip() function.